This invention relates to high power metal oxide semiconductor field effect transistors, and more specifically relates to a novel connection of the peripheries of the electrode pads of such devices directly to the underlying silicon to prevent failure of the device when it is operated in a diode mode.
High power metal oxide semiconductor field effect transistors (MOSFETS) which typically may be vertical conduction devices are well known. Commonly, such devices consist of a very large number of cells, which might be greater than 5,000, on a single small chip area with the devices all connected in electrical parallel. Typically, each cell may consist of a base region diffused into one surface of the main wafer or chip. A source region is formed within each base region and defines a conventional MOSFET channel. Each MOSFET channel underlies a conventional MOSFET gate which may be formed of polysilicon. The gate elements are connected together and can be connected to a common gate electrode pad, which is accessible on one surface of the chip, to which convenient gate connection can be made. Similarly, each of the sources is conventionally connected to a single sheet electrode which overlies the entire chip surface and which extends to a source electrode pad for making easy connection to the source of the device. The electrode pads are normally supported atop an oxide layer which is disposed between the pad surface and the underlying silicon wafer or chip surface. Conventionally, the silicon beneath the pad regions is of the same conductivity type of the cell base regions.
The source electrode which contacts each of the sources within each of the base regions also makes contact to a central portion of the cell base region. Consequently, the overall device acts as a single junction device or diode when the source electrode is one polarity, but as a MOSFET when the source electrode is of the other polarity. It has been found that MOSFET devices of the above structure experience failure during operation in a diode mode under particular circuit conditions. Upon inspection, it was discovered that failure occurred in the cell elements around the periphery of the electrode pads.